3rd gen iPad charging/fully charged

[Nick05]

I bought a 3rd generation iPad when it came out. Somewhere during the first year of using it, when it is fully charged it states "not charging" by the battery icon. I figured it might be something to do with a software update, but I recently purchased a second 3rd generation iPad and it just shows the fully charged battery icon. Do you think I might have a battery issue? I use the same charger for both so that rules that out.